Lysophospholipids are membrane-derived bioactive lipid mediators, of which one of the most medically important is lysophosphatidic acid (LPA). LPA is not a single molecular entity but a collection of endogenous structural variants with fatty acids of varied lengths and degrees of saturation (Fujiwara et al., J Biol. Chem., 2005, 280, 35038-35050). The structural backbone of the LPAs is derived from glycerol-based phospholipids such as phosphatidylcholine (PC) or phosphatidic acid (PA).
The LPAs are bioactive lipids (signaling lipids) that regulate various cellular signaling pathways by binding to the same class of 7-transmembrane domain G protein-coupled (GPCR) receptors (Chun, J., Hla, T., Spiegel, S., Moolenaar, W., Editors, Lysophospholipid Receptors: Signaling and Biochemistry, 2013, Wiley; ISBN: 978-0-470-56905-4 & Zhao, Y. et al, Biochim. Biophys. Acta (BBA)-Moil. Cell Biol. Of Lipids. 2013, 1831, 86-92). The currently known LPA receptors are designated as LPA1, LPA2, LPA3, LPA4, LPA5 and LPA6 (Choi, J. W., Annu. Rev. Pharmacol. Toxicol., 2010, 50, 157-186).
The LPAs have long been known as precursors of phospholipid biosynthesis in both eukaryotic and prokaryotic cells, but the LPAs have emerged only recently as signaling molecules that are rapidly produced and released by activated cells, notably platelets, to influence target cells by acting on specific cell-surface receptors (see, e.g., Moolenaar et al., BioEssays, 2004, 26, 870-881, and van Leewen et al., Biochem. Soc. Trans., 2003, 31, 1209-1212). Besides being synthesized and processed to more complex phospholipids in the endoplasmic reticulum, LPAs can be generated through the hydrolysis of pre-existing phospholipids following cell activation; for example, the sn-2 position is commonly missing a fatty acid residue due to deacylation, leaving only the sn-1 hydroxyl esterified to a fatty acid. Moreover, a key enzyme in the production of LPA, autotaxin (lysoPLD/NPP2), may be the product of an oncogene, as many tumor types up-regulate autotaxin (Brindley, D., J. Cell Biochem. 2004, 92, 900-12). The concentrations of LPAs in human plasma & serum as well as human bronchoalveolar lavage fluid (BALF) have been reported, including determinations made using sensitive and specific LC/MS & LC/MS/MS procedures (Baker et al. Anal. Biochem., 2001, 292, 287-295; Onorato et al., J. LipidRes., 2014, 55, 1784-1796).
LPA influences a wide range of biological responses, ranging from induction of cell proliferation, stimulation of cell migration and neurite retraction, gap junction closure, and even slime mold chemotaxis (Goetzl, et al., Scientific World J., 2002, 2, 324-338; Chun, J., Hla, T., Spiegel, S., Moolenaar, W., Editors, Lysophospholipid Receptors: Signaling and Biochemistry, 2013, Wiley; ISBN: 978-0-470-56905-4). The body of knowledge about the biology of LPA continues to grow as more and more cellular systems are tested for LPA responsiveness. For instance, it is now known that, in addition to stimulating cell growth and proliferation, LPAs promote cellular tension and cell-surface fibronectin binding, which are important events in wound repair and regeneration (Moolenaar et al., BioEssays, 2004, 26, 870-881). Recently, anti-apoptotic activity has also been ascribed to LPA, and it has recently been reported that PPARγ is a receptor/target for LPA (Simon et al., J. Biol. Chem., 2005, 280, 14656-14662).
Fibrosis is the result of an uncontrolled tissue healing process leading to excessive accumulation and insufficient resorption of extracellular matrix (ECM) which ultimately results in end-organ failure (Rockey, D. C., et al., New Engl. J. Med., 2015, 372, 1138-1149). Recently it was reported that the LPA1 receptor was over-expressed in idiopathic pulmonary fibrosis (IPF) patients. LPA1 receptor knockout mice were also protected from bleomycin-induced lung fibrosis (Tager et al., Nature Med., 2008, 14, 45-54).
Thus, antagonizing the LPA1 receptor may be useful for the treatment of fibrosis such as pulmonary fibrosis, hepatic fibrosis, renal fibrosis, arterial fibrosis and systemic sclerosis, and thus the diseases that result from fibrosis (pulmonary fibrosis-Idiopathic Pulmonary Fibrosis [IPF], hepatic fibrosis-Non-alcoholic Steatohepatitis [NASH], renal fibrosis-diabetic nephropathy, systemic sclerosis-scleroderma, etc.)